The present invention relates to a fastening enabling a plastics material part to be secured in slidable manner to a support, said part nevertheless being capable of being detached from the support if a large force is applied.
It is known that the expansion of bodywork parts made of plastics material gives rise to problems in the manufacture of vehicles because of painting and cataphoresis operations, and also in subsequent use because of post-shrinkage of the parts and because of the temperatures they can reach in the lifetime of the vehicle.
To resolve those problems, add-on fastenings are used that enable a bodywork part to slide relative to the structure of the vehicle. Such fastenings are referred to as xe2x80x9csliding fasteningsxe2x80x9d.
French patent application No. 96/04317, published under the No. 2 747 091 describes such a sliding fastening.
Nevertheless, another problem arises with fastenings for plastics material parts, particularly with bodywork parts for motor vehicles.
In addition to their tendency to expand relative to their support, such parts break on being subjected to a certain level of force, such that when used as bodywork parts they are advantageous for impacts at small speeds because the flexibility of the plastics material enables them to withstand small deformations without damage, however they can give rise to problems both in terms of repair costs and in terms of safety when impacts are of greater energy since a piece of broken plastics material can constitute a sharp object that is particularly dangerous.
The present invention seeks to propose a novel sliding fastening which presents the conventional advantages of such a fastening, and indeed others which are explained below, and which eliminates the above-mentioned drawbacks associated with the high forces to which plastics material parts fastened in this way can be subjected, and in particular so as to reduce repair costs after an impact at 15 kilometers per hour (km/h).
The present invention provides a sliding fastening for a plastics material part having at least one thing wall that is to be slidably fastened to a support via its thin wall, the part including in said thin wall an elongate opening having two longitudinal edges that are substantially parallel at least over a portion of their length, the fastening comprising a spacer that is separate from the part, that is thicker than the thickness of the thin wall, and that is suitable for sliding in the opening between the two longitudinal edges thereof, a cover completely covering the spacer, being larger than the opening so as to be incapable of passing through it in order to hold the thin wall in register with the thickness of the spacer, and a base for securing to the support, wherein the spacer and the base are united by frangible links.
In the invention, the term xe2x80x9cthin wallxe2x80x9d is used to designate a portion of the part having one dimension that is very small compared with its other two dimensions.
The sliding fastening of the invention is preferably mounted on the part to be fastened, thereby enabling the part to be handled with the fastening preinstalled for fastening to-the vehicle.
The frangible links of the fastening enable it to release the plastics material part when they are subjected to a high level of force, for example during an accident if they are on a motor vehicle.
The bodywork part is thus preserved since it becomes detached from the vehicle instead of breaking, thereby achieving savings in terms of vehicle repair, and also protecting the people involved in the accident from any risk of being cut by a sharp object of the kind that can be constituted by a broken piece of bodywork. This applies in particular to plastics material fenders for motor vehicles.
In a particular embodiment of the invention, the spacer and the base are united by links that are not only frangible, but also releasable.
The first lies in the fact that manufacture of the plastics material for the spacer and the base is made easier, in particular with respect to the frangible links which would be difficult to calibrate accurately if the spacer and the base were molded as a single piece.
The second advantage lies in mounting the fastening on the thin wall of the part. Because the links are releasable, it is possible to present the spacer and the base on either side of the thin wall and to assemble them together by means of the releasable links, with the thin wall being held captive in the thickness of the spacer.
In a particular embodiment, the releasable links are constituted by pegs which engage in oblong openings and which are held therein by relative displacement between the spacer and the base.
The pegs can be on the spacer and the oblong openings in the base, or vice versa.
The relative displacement between the spacer and the base is preferably a pivoting movement about a central axis of the fastening that is perpendicular to the plane of the thin wall.
In a particular embodiment, the spacer and the base are prevented from moving relative to each other by complementary shapes snapping into engagement, for example a projection on the base in a recess of the spacer, or vice versa.
The surface areas of the spacer and the base that bear against the support are preferably large enough to prevent any creep of the fastening, even when its temperature rises.
These surface areas are selected as a function of the desired conditions and of the material used.
Appropriately selected bearing surface areas make it possible in particular to avoid using a lock-washer of the kind usually employed for ensuring that a screw is kept tight when the spacer is secured to the support by means of a screw.
In a particular embodiment, the base has tongues for holding the thin wall in the thickness of the spacer, between the base and the cover.
In embodiments in which the links between the spacer and the base are not releasable, one of the longitudinal edges of the opening has one or more slots of shape corresponding to that of the tongue(s) of the base, enabling the spacer to be put into place in the opening. In addition, asymmetrical tongues can be provided on each side of the base so that the slots of the opening allow the spacer to be inserted in a single orientation only, wherever the spacer needs to comply with a particular orientation.
In a particular embodiment of the invention, the spacer has an elastically deformable portion that bears slidably against the thin wall of the part while the spacer is sliding inside the opening, the thin wall having a coupling shape organized in such a manner that when the spacer is in a privileged position relative to the opening, the deformable portion of the spacer couples with the thin wall.
In other words, while the spacer is sliding in the opening, the elastically deformable portion snap-fastens in the coupling shape of the thin wall, thereby defining a hard point in the stroke of the spacer, corresponding to the privileged position for the spacer relative to the opening.
This privileged position is defined as being a prepositioning position for the spacer on the part, in order to facilitate assembling said part on its support.
For example, for a motor vehicle bodywork part, the privileged position of the spacer relative to the opening is such that the spacer can move inside the opening in a first direction to compensate post-shrinkage of the thermoplastic material, and in a second direction, opposite to the first, to compensate differential expansion between the thermoplastic material part and the structure of the vehicle.
Since expansion differences are preponderant, the stroke available to the spacer in the opening in the second direction is preferably greater than that made available to it in the first direction.
When one of the longitudinal edges of the opening has one or more slots for installing the spacer fitted with tongues in the opening, the position in which the spacer is placed in the opening is preferably at a distance from its privileged position.
In a preferred variant of this embodiment, the elastically deformable portion of the spacer is constituted by a resilient tab provided with an external bulge which bears against one of the longitudinal edges of the opening, the longitudinal edge in question having as its coupling shape a groove into which the bulge of the resilient tab engages when the spacer is occupying its privileged position relative to the opening.
In a preferred embodiment of the invention, said elastically deformable portion of the spacer is directly molded with the spacer, e.g. the resilient tab provided with a swelling is molded directly with the spacer.